Timing considerations are important in many digital circuit applications. A circuit functioning under one clock rate may be required to receive and operate on data transmitted at another clock rate. At other times, a circuit may be required to receive and operate on data transmitted at the same clock rate but aligned to an unknown phase, or what is known as plesiochronous data. In these applications, the circuit may be required to capture the data under unstable conditions in which there are no setup or hold requirements for the input data relative to the internal circuit clock.
One way to achieve phase alignment in these applications is to oversample the incoming data according to a Nyquist criteria. A serious disadvantage associated with oversampling is the amount of logic circuitry required to process and store the large quantity of sampled data. In most applications, the large amount of required circuitry presents a formidable obstacle. The inadequacy of using oversampling to achieve phase alignment is especially apparent when incoming data is arriving at a fast rate.
In the telecommunications field, telephony, digital, and video data are transmitted and switched by telephone networks at fast data rates. These data may be in any data format, such as DS1, DS3, T1, STS-1, and SONET. In order to properly switch, transmit or perform other data transformation processes, the plesiochronous data must be captured and transformed to one that is synchronous and phase-aligned with respect to a selected clock signal. For example, a crosspoint or crossconnect switch may be required to receive and switch plesiochronous data under unstable conditions prior to switching and outputting the switched data to the telephone network.
In addition to the plesiochronous nature of the data, long cables interconnecting the various communications components in the system may introduce timing delays and variations. For example, the physical distance between an originating stage or a terminating stage and the crosspoint switch may be on the order of one hundred and fifty feet or more. Further timing problems, such as jitter, wander, and drift inherent in telephony data, are introduced by large ambient temperature variations and aging of the system components. When substantial shifting of data with respect to the clock occurs, data bits are essentially deleted or added, depending on the direction of the data shift. If left uncorrected, such data shifts may result in erroneous or difficulty in embedded bit pattern framing downstream. Therefore, to eliminate or greatly reduce error in the transmitted data, these timing problems must also be resolved.
Accordingly, a need has been identified for a circuitry or method to capture and realign plesiochronous data. Furthermore, timing and framing problems arising from data drifting, jitter, and wander also require adequate resolution.